The cellular radiotelephone system uses numerous antennas located throughout a metropolitan region. Each antenna is typically at the center of th a cell that is divided into six 60.degree. sectors. Each sector has a group of frequencies that are transmitted at low power in order to communicate with mobile or portable radiotelephone that are within the sector. When the radiotelephone moves from one sector to another, the frequency being used is automatically switched, allowing the telephone call to continue uninterrupted. This permits a radiotelephone user to remain in contact with the landline telephone system anytime the radiotelephone is within range of a cellular radiotelephone system.
The popularity of this type of communication has caused some cities to experience an overload of radiotelephone calls. There are times when all frequencies are being used and additional users must wait to access the system until a frequency is free. This has led the cellular industry to develop a new generation digital cellular radiotelephone system using time division multiple access (TDMA) to replace the current analog system.
The TDMA system divides each frequency into times-lots, thus allowing a number of radiotelephones to use a frequency simultaneously. The DTMA system, therefore, greatly expands the number of radiotelephone that can use the radiotelephone system.
Mobile radiotelephones can be used with a handset or in a hands-free mode. The handset has a microphone and speaker and is held close to the face to communicate. In a TDMA system, the microphone is typically connected indirectly to a vocoder that converts the analog voice signal into codewords that are a quantized parameter representation of the speech signal.
Hands-free operation allows the user to communicate a short distance from a microphone and speaker without holding a handset. For proper hands-free operation, the microphone audio must be attenuated when the speaker is operating. This reduces the speaker audio that is receioved by the microphone and transmitted back to the person speaking at the other end of the call. Without the attenuation, the person on the landline end of the call would experience an acoustic echo of their own voice. This echo is delayed by the time it takes for the voice signal to travel through the system and return to the originator.
Depending on the volume of the speaker audio, the attenuation may not be enough to remove the acoustic echo. Radiotelephone systems, therefore, may have an acoustic echo canceller to remove more of the echo. Again, depending on the speaker audio volume, even this may leave enough of the echo to be detectable. There is a resulting need for a method to completely eliminate the acoustic echo.